The fission product release and transport was analyzed in unit 3 of the Fukushima Daiichi nuclear power plant using severe accident code MELCOR2.1. A best estimate accident sequence was re-constructed as a part of the OECD/NEA project "Benchmark Study of the Accident at the Fukushima Daiichi Nuclear Power Plant (BSAF). For this sequence, the fission product release from the fuel and transport in the plant as well as release to the atmosphere were evaluated. The fission product concentrations in the containment drywell and suppression chamber were compared with the dose rate measurements with the Containment Atmosphere Monitoring System (CAMS) during the times when measurements were available. In addition, the activity release to the environment was compared with the activity release determined using methods based on environmental measurements, distribution models, and observed plant events (WSPEEDI).

Further analyses were carried out to refine the accident sequence. The analyses were conducted for 350 hours from the accident initiation. Three additional cases were calculated by varying the coolant water flow into the reactor pressure vessel, and the timing of the containment venting actuations. Comparison with the plant data showed that the probable accident progression could be found within the envelope of the two out of the three sensitivity cases. The results of the BSAF best estimate and the additional sequences are discussed with the emphasis on fission product transport.